Bridge plug assembly



H. E. NASH 2,32,8U

BRIDGE PLUG ASSEMBLY Filed May 25, 1940 HENRY E.

INVENTOR.

Patented June 1, 1943 BRIDGE PLUG ASSEMBLY Henry E. Nash, Saranac Lake,N. Y., assignor to Hercules Powder Company, Wilmington, Del., acorporation of Delaware Application May 25, 1940, Serial No. 337,211

4 Claims.

This invention relates to an improvement in resistance wire for electriccurrent and more particularly to resistance wire for use in electricfiring devices, especially as the bridge wire, in bridge plug assembliesused. t6 initiate electric blasting caps, squibs, igniters and the like.

It is well known that electric firing devices include an initiatingmeans normally comprising a bridge plug, cavity plug, or match headassembly which contains a high resistance bridge wire or platinum orplatinum alloyed with iridium, rhodium, rubidium or similar metals toeffect ignition of the device. Bridge wires of gold, gold alloys, suchas white gold, nickel-copper alloys such as constantan and some ironalloys have also been used, but the platinum alloys are most generallyused in the art as they exhibit marked resistance to corrosion.

The precious metal bridge wires are very expensive to use while thenickel-copper and iron alloys may not provide sufiicient stability whenin contact with the air and with certain explosive compounds used inelectric firing devices. I

Further, the thermic coefiicient of the known bridge wire materials arenot the most desirable, and also the action upon the application of asource of E. M. F. to these materials is not always predictable andconstant. Thus, the bridge wire may, upon the application of E. M. F.,fuse and break the circuit causing other devices in the series to fail,or it may not rise to a temperature sulilciently high to be efiective inthe ignition of the charge in the device.

The object of this invention is to provide a bridge wire for electricfiring devices which will emciently initiate the device.

A further object of this invention is to provide a bridge wire forelectric blasting caps which will positively initiate the explosivecharge thereof.

The further object of this invention is to provide a, bridge wire forelectric firing devices which does not depend entirely upon the heatgenerated due to the application of a source of E. M. F. to initiate thedevice.

A still further object of this invention is to provide a pyrophoricbridge wire for electric blasting caps. v

Other and further objects will become apparent hereinafter.

The objects of this invention have been accomplished by providing bridgewire for electric firing devices comprising zirconium which is drawninto a high resistance wire of proper diameter to provide the electricalresistance per unit length desired for the particular use for which thebridge wire is intended.

As illustrative'oi the use of zirconium resist.

ance wire embodying this invention, for example, as a bridge wire forelectric blasting caps, the wire will be'made of a good grade of ductilezircom'um, and will be drawn to a diameter of between about .0010 inchand about .0029 inch as this diameter has an electrical resistance orbetween about 300 ohms and about 4&0 ohms per yard. The preferablediameter is about .0013 inch which gives a resistance of about 360 ohmsper yard. The proper length of wire is secured to the usual leg wires orthe bridge plug assembly of an electric blasting can by welding,swaging, or in a similar manner, and the bridge wire terminals thenslightly crimped towards each other to relieve the tension on thezirconium wire. The bridge plug assembly is inserted into contact withthe charge within the device to be initiated, for example, into apriming charge of diazodinitrophenol in an electric blasting cap. Thebridge plug assembly is sealed into the device and is then ready toefi'ectively initiate the charge contained therein. Upon the applicationof a source or E. M. F. to the leg wire, the zirconium bridge wirebecomes sufiiciently hot to cause initiation of the charge contained inthe electric firing device.

A preferred embodiment of my invention will be described withreferenceto the accompanying drawing in which the single figurerepresents an electric blasting cap.

The electric blasting cap shown in the figure comprises an explosivecharge i in the closed end of a casing 2 The charge I has superimposedthereon an ignition means comprising a plug 3 having a pair of leg wires4 extending therethru and a bridge wire 5 electrically connected acrosstheir terminals. This bridge wire 5 in accordance with this invention iscomposed of zirconium. The ignition device is maintained in position bya water-proofing materal 6 and a sealing compound l'.

The explosive charge i may be comprised of a single charge of primaryexplosive, for example, mercury fulminate, diazodinitro phenol, leadazide or the like, alone or in admixture with an oxidizing agent such aspotassium chlorate, barium peroxide, lead dioxide or the like, or it maybe comprised of base charge of tetryl, TNT, nitrostarch, pentaerythritoltetranitrate, or the like with a primary charge of the above mentionedtypes of explosives superimposed thereon.

While pure zirconium wire is much easier to draw to the proper diameterto obtain the correct electrical properties, the use of those zirconiumalloys which may be drawn to the desired form of wire and which retaintheir pyrophoric nature is contemplated by this invention.

The initiation of the electric firing device, discussed above. is aidedby the pyrophoric nature of the zirconium bridge wire. Thus, thezirconium, upon arriving at a dull red heat, suddenly fiashes with avery intense white light that is extremely hot. This pyrophoricphenomenon increases the efficiency of the firing device and also aidsin firing or initiating the charge within the device. Thus, inaccordance with this invention, a more eflicient firing device, forexample, electric blasting cap may be produced.

A further advantage of this invention is in firing a large number tcaps. Thus, plurality of caps are commonly fired by connecting them inseries to a source of power. Upon application of the power, the currentpasses through all the caps in the series, heating the bridge wire ineach at about the same rate. The bridge wire reaches the ignitiontemperature of the surrounding ex plosive in perhaps .005 second, but itis usually necessary to rise considerably above the ignition temperatureto surely eifect ignition. Because of variations in the explosive andperhaps a slight non-uniformity in the manufacture of the cap, not allcaps are equally sensitive and there is a tendency for some of the capsto fire before the others. It will be, of course, apparent that when thefirst cap fires, its bridge wire is destroyed and the circuit is opened,so that all heating of the remaining bridge wires ceases. They proceedto cool, transferring their heat to the explosive, which will generallyenable these caps to likewise fire. However, it sometimes happens thatthe heat generated by the current is insufficient to fire a particularlyinsensitive cap. The result is a failure of the dynamite primed by thiscap to explode, and the misfire may have very damaging consequences.This condition is aggravated by low currents.

When zirconium bridge wires are used, about the same sequence of events,up to the point of the firing of the sensitive caps and the cooling ofthe remaining bridge wire, takes place. These bridge wires, however,have reached the pyrophoric ignition temperature of the zirconium, andthe zirconium proceeds to burn to zirconium oxide, raising thetemperature and evolving considerable heat. It is calculated that undera normal set of conditions, the heat thus created may amount to over tentimes the heat generated by the electric current. The danger of failureof an insensitive cap may thus be reduced or largely eliminated, andlower currents may be used to fire the caps without increasing thelikelihood of misfires. Many more caps may be used in a seriesconnection, and thus numerous improvements in field performance due tothe improved reliability of of material. However, when using zirconiumbridge wires of this invention, the small amount of material is ignitedby the flash produced when the zirconium is raised to its pyrophorictemperature.

The use of zirconium bridge wires of this invention in match headassemblies has been found to be highly desirable because the match headassemblies now known to the art are not regular in firing time. Thezirconium bridge wire, due to the pyrophoric nature allows theproduction of more uniform match heads thereby giving more satisfactoryfiring devices.

The pyrophoric nature of the zirconium or zirconium alloy bridge wire ofthis invention has been found to be enhanced by coating the bridge wirewith an oxidizing agent, for example, lead dioxide, barium peroxide,potassium chlorate, etc. The coating is aided by the use of a bindersuch as nitrostarch, nitrocotton, or the like in very small amounts. Bythe use of the oxidizing agent, a more brilliant fiash is produced andbetter ignition of charges in the firing device is obtained. In someinstances the inclusion of minor amounts of flame-producing materials inthe coating has proved advantageous. Flame-producing materials of. acarbonaceous nature such as, for example, charcoal or the like may beused.

It will be obvious to those skilled in the art that zirconium bridgewire alone or with oxidizing agents, binders, and flame producing agentssurrounding it in accordance with the foregoing description of thisinvention may be used in bridge plug assemblies of variou type firingdevices such as, for example, electric blasting caps, delay electricblasting caps, squibs, igniters and other special caps, whether thesecaps contain a bridge plug assembly that has two terminal wireselectrically connected by a zirconium bridge wire that is inserteddirectly into the explosive charge of a firing device as describedhereinbefore with referenc to the single figure;

or whether the bridge plug assembly is of thecavity type having theterminals of the wires in a recess so positioned that an ignitioncompound is pasted into the recess and around the bridge wire; orwhether the bridge plug assembly is a match head having a sensitiveexplosive pasted around the bridge wire. In each of the above type capsand bridge plug assemblies, the zirconium bridge wire of this inventionprovides an increased efliciency over the use of platinum bridge wires.

It will be understood that the details and examples given hereinbeforeare illustrative only and in no way limiting on my invention as broad lydescribed herein and in the appended claims.

What I claim and desir to protect by Letters Patent is:

caps containing zirconium bridge wires are obtained. The theorydiscussed above is believed to be correct, but this invention should notbe construed as limited thereby.

Another advantage of this invention isthe positiveness by which thezirconium bridge wire ignites ignition materials which are difficult toignite with an electric bridge wire assembly which has a platinum bridgewire. For example, in using cavity plugs, only a small amount ofignition material is placed within the cavity around the bridge wire,and oftentimes the heat of the platinum bridge wire fails to ignite thissmall amount 1. An electric firing device of the character described,including, in combination, a casing, an explosive charge and a firingassembly adjacent said charge, said assembly comprising a pair of legwires disposed in spaced relationship to each other, a bridge wireelectrically connecting said leg wires, said bridge wire being made ofzirconium and having a metallic oxide adhering to said wire whereby saidbridge wire is caused to be pyrophorized under a relatively lowelectrical potential during a, relatively short period of time.

2. An electric firing device of the character described, including, incombination, a casing, an explosive charge and a firing assemblyadjacent said charge, said assembly comprising a pair of leg wiresdisposed in spaced relationship to each other, a bridge wireelectrically connecting said leg wires, said bridge wire being made ofzirconium and having a metallic oxide secured directly to said wire bymeans of a binder whereby said bridge wire iscaused to be pyrophorizedunder a relatively low electrical potential'durlng a relatively shortperiod of time.

3. An electric firing device of the character described, including, incombination, a casing, an explosive charge and a firing assemblyadjacent said charge, said assembly comprising a pair of leg Wiresdisposed in spaced relationship to each other, a bridge wireelectrically connecting said leg wires, said bridge wire being made ofzirconium and having a metallic oxide secured directly to said wir bymeans of a nitrostarch binder whereby said bridge wire is causedto bepyrophorlzed under a relatively low electrical potential during arelatively short period of time.

, said charge, said assembly comprising a pair of leg wires disposed inspaced relationship to each other, a bridge wire electrically connectingsaid leg wires, said bridge wire being made of zirconium and having ametallic oxide secured directly to said wire by means of a nitrocottonbinder whereby said bridge wire is caused to be pyrophorized under arelatively low electrical poteritial during a relatively short period oftime.

HENRY E. NASH.

